Alot of it has to do with moderation. If you don't perform aswell or aren't as motivated at work/school when your high then it's best to wait till the end of the day or for the weekend and use it as a reward for your hard days/weeks work.

For me anyways I have found that it takes away some of my hunger, drive and ability to focus clearly on my personal and proffesional goals and seeing those goals through. So for me anyway I wait till nightime and sometimes I go a few days without but there are more important things than weed and if it is holding you back by all means take a break and moderate.